1. Field of the Invention
The present invention relates to a shifting-transfer mechanism and, more particularly, to a shifting-transfer mechanism suitable for transferring between two-heel drive and four-wheel drive in a four-wheel drive vehicle and also for changing between high- and low-speed gear ratios of a power train from a transmission to a drive axle.
2. Description of Related Art
As a 2WD/4WD transfer mechanism for transferring from two-wheel drive to four-wheel drive in, for instance, a four-wheel drive vehicle, a transfer mechanism is known where transfer between four-wheel drive and two-wheel drive is made by connecting and disconnecting a slider with splines. Here, splines are formed on the front and/or rear axle shaft. Normally, during two wheel drive mode, only one of the axles acts as the drive axle, but during four wheel drive mode, both axles are normally constantly or intermittently driven by engine power via the transmission.
Also in HIGH/LOW shifting mechanisms for changing between a high and low speed gear ratio for a power train from the transmission to the drive axle, mechanisms are known where the gear ratio of the power train from the transmission to the drive axle is set to either HIGH range or LOW range by connecting the drive axle splines to the output shaft splines of the transmission. Alternatively, such a device mates the drive axle splines in such a way so that the torque is transmitted from the transmission through a speed changing mechanism. This HIGH/LOW shifting is accomplished by moving the slider, engageable with the splines, along the drive axle axis. The HIGH/LOW shifting mechanism is designed to change the gear ratio according to a control command from the driver during four-wheel drive of the motor vehicle. Generally during two-wheel drive, the gear ratio is locked in HIGH to maximize fuel economy.
A thrust actuator has been used for driving the 2WD/4WD transfer mechanism and the HIGH/LOW shifting mechanism as stated above. This thrust actuator slides the slider from being connected to the splines to being disconnected (slid in an opposite direction) from the splines. Here, a shift fork, formed on a fork shaft, changes motor rotation into a linear motion along the axial direction of the rod. Then, the fork shaft, locked on the forward end of the rod, is shifted.
Recently, some vehicles with part-time four-wheel drive systems employ a shifting mechanism where a single actuator is used to make a 2H-4H-N-4L shift. Such a shift is to two-wheel drive high speed (2H), four-wheel drive high speed (4H), neutral position (N) and four-wheel drive low speed (4L). This mechanism has fewer components than one having a plurality of actuators. Accordingly, this device is mountable in a narrow mounting space, thereby reducing manufacturing cost.
With a single actuator for 2H-4H-N-4L shifting, the vehicle can safely be driven essentially without changing vehicle speed when a 2H-4H shift is made while moving. However, if a 4H-N-4L shift is made during driving, the vehicle speed will suddenly change because there is one position in the shift pattern where the driving power decreases. Normally, therefore, the 4H-N-4L shift is done when the vehicle is stopped. However, if the 2H-4H shift is made during driving, failing to detect the 4H position and further operating the actuator in the range of N or 4L may decrease driving power and cause a change in vehicle speed.
However, a method for detecting a gear position switch failure is disclosed in JP-A No. S63-34349. Here, each gear of an automatic transmission is shifted by a gear shifting means. A gear position is detected by a gear position switch that determines a failure of the gear position switch to detect a target gear position when no shift is completed, even by a retry. This failure method detection is suitable for determining a failure of the position detecting switch in movement between two points.
However, in the shifting mechanism having a single actuator for performing the 2H-4H-N-4L shift, it is necessary to stop at a point between a 4H-N shift. If the method disclosed in JP-A No. S63-34349 is adopted, the actuator may be driven past the target position to the range of N or 4L, when the target is set at a position between 4H-N, potentially resulting in decreased vehicle control.
There is a mechanism disclosed in Japanese patent application No. 11-278814. In this mechanism, four independent contact-type position detecting switches detect switching positions to drive an actuator.